Unrestricted file upload vulnerability in WordPress before 2.2.1 and WordPress MU before 1.2.3 allows remote authenticated users to upload and execute arbitrary PHP code by making a post that specifies a .php filename in the _wp_attached_file metadata field; and then sending this file's content, along with its post_ID value, to (1) wp-app.php or (2) app.php.
http://www.securityfocus.com/bid/24642
http://www.buayacorp.com/files/wordpress/wordpress-advisory.html
http://trac.mu.wordpress.org/changeset/1005